The invention relates to semiconductor processing systems, and, in particular, to devices and methods for controlling rapid thermal process at low temperatures.
Some semiconductor fabrication processing techniques, such as rapid thermal processing (RTP), require control of the wafer substrate temperature. In one system, the temperature is monitored during an RTP process by measuring the black body radiation emitted from the back side of the wafer while the wafer is being heated by an array of lamps illuminating its front side with broad-band light. The wafer is also rotated at about 90 RPM during the process to improve uniformity. More than one backside emission detector is typically employed, with a central detector positioned on the central axis of the spinning wafer during processing. The detectors are sensitive to a narrow band (about 40 nm) of radiation at about 950 nm wavelength. They typically are effective for monitoring and controlling temperatures above about 300-325.degree. C. Below that temperature there is an instrumentation cut-off because there is not enough emission from the wafer at the wavelength at which the detectors are sensitive to provide an accurate measurement. The wafer temperature is controlled by adjusting the illumination from the lamps with a feedback loop from at least one of the back side detectors. A description of an RTP system of this type is provided in co-owned U.S. Pat. No. 5,660,472, to Peuse et al., the entire disclosure of which is included herein by reference.
A problem can arise in the beginning of a heating cycle. Some wafer materials, such as undoped or lightly doped silicon, will transmit infrared radiation from the lamps at relatively low process temperatures that are above the cut-off temperature of the detectors. This transmitted radiation overwhelms the emitted blackbody radiation when the wafer is cold, rendering the monitor reading meaningless. As the wafer heats up, the wafer transparency diminishes and the blackbody radiation increases. Eventually, as the wafer becomes opaque, the blackbody radiation becomes the dominant portion of the detected signal. The temperature at which the wafer becomes opaque enough to not seriously disturb the temperature measurement and control will be about 500-800.degree. C., depending upon the doping of the wafer.
It is important to know the emissivity of a black body to calculate the body's temperature from its emissions. One method of making corrections to the emissivity of the substrate is described in U.S. Pat. No. 5,660,472. Another system optically detects a 60 Hz signal in the lamp output to make an adjustment of the emissivity at high temperatures, after the substrate becomes opaque.
A wafer's low temperature transparency can be a serious problem for processes on bare wafers that have no metalization on them. The solution to this problem has been to heat the wafer in an open loop mode, at constant power, until the back side measurement passes a measured minimum T.sub.m and a subsequent threshold value T.sub.t. Then the feedback loop is employed to control the temperature. The open loop method of heating is illustrated in FIG. 1. Even with this method, there is no accurate temperature measurement from the backside detector, and therefore no temperature control, until the wafer reaches a temperature at which it becomes substantially opaque.
For most RTP processes, however, the wafers will have patterns of oxides and metalizations on them that absorb or block the lamp illumination from being transmitted. There may be some transmission, but not to a large enough extent to seriously affect the temperature measurement and control. Because the illumination from the lamps is blocked from reaching the back side emission detectors, temperature control can be effective from lower temperatures, even down to a temperature at the sensitivity limit of the detectors.
In a typical RTP facility, a cassette of several wafers, for example 25 wafers, is processed as a group through a predetermined processing recipe. Periodically, for example, twice a day, the manufacturer will double check its processes to make sure all the processes are still running within statistical control. The manufacturer will do the check by running a so called monitor wafer through the process. A monitor wafer is typically bare on the top; it does not have a pattern of oxides and metalizations to block the lamp illumination. The monitor wafer goes through the same recipe as the product wafers. Afterward, the monitor wafer is examined. If the monitor wafer is not processed by the same recipe as the product wafers, then the monitor wafer can no longer be predictive of the actual process at these low temperatures. On the other hand, if the process is changed to accommodate the monitor wafers, then the process is less efficient than it would otherwise be with temperature control at lower temperatures. Lack of temperature control at low temperatures impacts throughput and impacts process control.